This invention relates to a system and a method for measuring crude oil, and, more particularly, to a system for accurately measuring oil as it is transferred from a lease storage tank to a transport vehicle.
Currently, oil extracted from the ground is stored on the lease site in one or more storage tanks, often collectively called tank batteries. These storage tanks typically are large cylindrical vessels into which the oil is pumped via a pump jack or other extraction structure. Periodically, the oil must be transferred from the storage tanks to a refinery or central storage area. Usually, the oil produced on a particular lease is the property of a particular producer and landowner. The oil is typically purchased from these entities by a refining company. The refining company collects the oil from the leaseholds via transport tanker trucks. As is apparent, it is necessary for the refining company to keep track of the amount of oil obtained from a particular leasehold so that the lease producer and landowner can be adequately compensated.
Currently, oil is collected in the same manner it has been for the last fifty years. A transport truck is pulled close to a storage tank and coupled thereto via a valve system. Prior to loading, however, certain parameters are needed from the storage tank. More specifically, tables are available for the cross-sectional size of the tank such that the volume of oil in the tank can be computed by measuring the height of the oil prior to loading and the height of the oil after loading is completed. However, other factors can also affect the volume of the oil transferred from the tank, such as its density and temperature.
In a typical loading process, a tanker truck operator prior to transferring oil from the storage tank to the tanker truck will take temperature, gravity (density), height, encrustation readings and basic sediment and water content. More specifically, prior to the loading onto the transport vehicle, an operator must climb to the top of the tank to take various readings from the oil inside. Tanks usually have hinged vent lids on the top which allow the operator access to the interior of the tank from the upper surface of the tank. A ladder and/or catwalk structure is utilized to allow the operator access to these vents. Therefore, an operator must climb to the top of the tank to take these readings. A temperature reading is taken by lowering a wood back thermometer to the center of the tank for approximately fifteen minutes and then retracting the thermometer and noting the temperature reading. Thus, the temperature only is taken at one particular level in the tank. Unfortunately, there can be various temperature gradients within the tank, for instance, depending upon whether the temperature is taken on the sunny or shady side of the tank. This is exacerbated by the fact that the access vents discussed above are typically adjacent one edge of the tank.
Further, while a temperature reading is being taken, a sampling device known as a "thief" typically will be lowered into the tank to a level adjacent the bottom of the tank to take an oil sample which is then retrieved from the tank indicating the bottom in the tank. The thief also will be lowered into the tank at predetermined levels to detect the basic sediment and water content of the oil and to define the observed gravity. Thereafter, the operator will lower a hand-operated gauge line into the tank to determine the height of the column of oil in the tank. The operator will then climb down from the tank and proceed to the cab of the transport truck with the gathered information. The oil sample is analyzed by the operator to determine the density of the oil in the tank. Further, the oil sample is centrifuged to get a representative reading of the sediments and water in the tank. Additionally, when the operator is on the tank, the level of encrustation on the tank wall adjacent the vent lid is noted. More specifically, the amount of buildup (such as rust) on the wall of the tank is noted. Thereafter, the operator leaves the cab of the truck and begins to unload the oil from the storage tank to the tanker truck.
After the tanker truck reaches a full level, loading is stopped. The operator then returns to the top of the tank and takes another oil level reading utilizing the gauge line. The operator also takes the observed temperature and checks bottom levels. The operator then returns to the truck with this last piece of information, and, thereafter, computes the net volume of oil transferred utilizing the temperature reading, density reading, oil level difference, sediment and water readings, and encrustation value. As is apparent, there are numerous inaccuracies associated with this collection method. First of all, the temperature is taken at only one level when there can be numerous temperature gradients throughout the oil contained in the storage tank. Further, the sample utilized to determine gravity and the sediment and water values is also only taken at predetermined levels depending on tank size. These values can also vary greatly throughout the volume of oil contained in the storage tank. Still further, the encrustation value taken at the vent lid is utilized in conjunction with a table to determine the approximate amount the volume should be reduced due to encrustation throughout the entire tank. As is apparent, this one value from one particular point in the tank does not provide a very accurate reading as to the amount the volume should be decreased due to encrustation. A further error in the readings results from the adhesion of the oil to the inside surface of the storage tank. More specifically, as oil is removed from the storage tank, a certain amount of oil will adhere to the tank's inner surface as the oil is removed from the bottom of the tank. This oil adhered to the sides will take a substantial amount of time to return to the main body of oil. Therefore, because the second depth reading is taken immediately after loading, a substantial amount of the oil adhered to the side has not returned to the main body of oil. Thus, this oil is computed as being removed from the tank when in fact it has not been removed.
A further disadvantage of the current transfer method involves the collection of oil having a high sulfur content. More specifically, with oil having an H.sub.2 S content of 300 parts per million (ppm) and higher, current safety standards require that at least two tank truck operators be present during collection. More specifically, because an operator has to climb up onto the tanks and physically have access to the interior of the tanks, the possibility exists of the operator being overcome by fumes from the tank. Therefore, while one operator is on the tank, another must remain at ground level. Further, because of the high sulfur value, the operator accessing the tank interior must don appropriate safety equipment, such as masks and gloves, each time access to the tank is desired. Therefore, the operator must don the safety equipment prior to initially climbing up onto the tank, and must further don the equipment later when the final height value is taken from the tank. Therefore, the donning and removal of the safety equipment adds additional time to an already cumbersome and time-consuming transfer process.
As is apparent, the current transfer system has numerous inaccuracies associated therewith which can result in mistrust between the refining company and the producer/landowner. Further, the steps of the current transfer procedure are very cumbersome and require a substantial amount of time and effort on the part of the persons collecting the oil. Therefore, a novel crude oil transfer system is needed which alleviates the above-discussed drawbacks of the current transfer procedures.